Method for heating a thermoplastic thread



United States Patent O U.S. Cl. 219-10.41 7 Claims ABSTRACT F THEDISCLOSURE Method for the reduction of the residual shrinkage ofthermoplastic threads wherein the thread wound on a supporting member issubjected to an RF electric field.

This invention relates to a method for heating a thermoplastic threadand, more particularly, to a method for reducing the residual shrinkingof a thermoplastic thread wound on a supporting tube in the form of aspool. The invention is particularly applicable to thermoplastic threadsmade of linear polyamides, such as those obtained by the condensation ofaminocarboxylic or bicarboxylic acids with diamines, or those made withlinear polyesters such as polyethylene terephthalate. It is known thatthreads made with linear polymers as mentioned above, such as obtainedwith the melt spinning method and subsequent drawing in the solid state,have usually an intrinsic shrinkability, or residual shrinking, whenthey are treated with water which has been heated to a temperature inthe neighbourhood of 100 C. This is tantamount to saying that a giventhread sample, immersed in hot water in a relaxed condition, willexhibit a shrinking in the longitudinal direction.

Such a shrinkage is a considerable disadvantage, in that it reduces thetensile strength of the thread and gives rise to an uneven molecularorientation. In addition, the fabrics prepared with the highlyshrinkable threads aforementioned, when they are subjected to heattreatments with steam, or to washing treatments at high temperatures,undergo an undesirable reduction of their dimensions." To do away withthis drawback, several methods have been suggested for reducing thethread shrinkage beforehand, that is, prior to employing it in fabrics.Two of the generally adopted methods to this purpose consist ofsubjecting the thermoplastic thread, immediately upon drawing, to a heattreatment by causing it to slide over a pin or a similar heating member,while the length of the thread is kept constant. The thread treated inthis manner is then collected on the usual spool, from which it can bedrawn and processed in the usual way. Even though this method is afairly satisfactory solution from an economical standpoint, in that itpermits that the conventional machinery may be used with appropriatemodications, it is nonetheless only partly satisfactory, inasmuch as theresidual shrinkage is still rather high.

Thus, for example a Nylon 6 thread, after the treatment recalled above,still retains a residual shrinkage of about 7%, whereas a still lowerValue would be desirable.

On the other hand, a lower residual shrinkage could be obtained bysubjecting the thermoplastic thread to particular treatments, such as aheat treatment with water or a treament with steam under pressure in anautoclave. In order to carry out these treatments, however, it isnecessary to transfer the thread from the spool as obtained with thedraft process to another package Which is adapted to said treatments. ltis obvious, then, that a method of this kind, even capable of givingsatisfactory results, is rather cumbersome and expensive.

Patented Dec. 23, 1969 ICC It has now been found that a satisfactoryreduction of the residual shrinkage of a thermoplastic thread can beobtained, especially a thread made with linear polyamides or linearpolyesters, wound on a tubular supporting member in the form of a spool,if the supporting member for the wound thread is used as the dielectricbetween two electrodes energized by an RF voltage, having an RMS valueranging between 1,000 and 2,000 volts and a frequency comprised between10 and 200 megacycles and a service capacity comprised between 2 and 6kilowatts for a time from 1 to 10 minutes.

The thread is thus subjected to the influence of an RF eld and becomesgradually heated, so that it undergoes a heat treatment while its lengthremains substantially constant. The spools of thread to be subjected tothe treatment can be the same as obtained through the performance of thedrawing process, and this fact is a considerable advantage of theinventive method. After the treatment, the thermoplastic thread can beused as such by taking it out of the spool. Its residual shrinkage willbe in the range from 1% to 4% according to the intensity of thetreatment and the nature of the polymer. It should be noticed that, withthe inventive method, the heat treatment of the thread spool takes placein a uniform manner throughout the entire mass of the thread, that whichalso ensures a regular trend of the shrinkage throughout the entirelength of the thread. A heat treatment which, instead, should be carriedout with an external heat source, such as subjecting the thread spool tothe action of infrared radiations, would give rise to a temperaturedifferential within the thread mass, in that the outermost layer of thespool would be heated more rapidly than the inner layers, the resultbeing an unevenness of the residual shrinkage throughout the entirethread length. In addition, on account of the poor thermal conductivityof polyamides and polyesters, this indirect heating method would berather slow.

As a generator of the RF field and conventional device can be used,provided, of course, that it has the specifications indicated above. Thefield electrodes should be so shaped as to match the particular form ofthe thread spool to be treated.

Thus, if the thread spool has been formed on a metal supporting tube, asis the case when the as drawn thread is collected with the dressing ringmethod, then the supporting tube itself can be used as either eldelectrode, Whereas the second electrode will be a tube having a diameterslightly greater than the overall diameter of the thread spool. Themetal tube which supports the thread spool, as it should be used as anelectrode, will be preferably made of aluminium. It is possible,however, to use tubes made of other metallic materials, for example,steel. It should be born in mind, however, that materials such as steel,when they are used as electrodes and are energized by an RF voltage,tend to become rapidly heated, a fact which could be conducive todrawbacks of various nature.

At any rate, the second electrode will be preferably made of copper.

If, conversely, the thread spool has been formed on a non-metalliccylindrical supporting member, for example, paper or a synthetic resin,as is the case when the collection of the as drawn thread is carried outon a rotary drum into which the supporting member has been slipped, thentwo confrontingly mounted electrodes will be required, placed in contactwith the two sides of the thread spool and preferably made of copper.

Be it understood, in any case, that thread spools having a shape otherthan that described herein can be subjected to the inventive treatment:consequently, field electrodes having a configuration suitable for suchforms of packing will be used without departing from the scope of thisinvention.

The invention will be now explained with referenceto the accompanyingdrawings which are not to be construed as limitations.A

FIGURE 1 is illustrative of an embodiment of the inventive method, inwhich the thread to be subjected to the action of the RF iield is woundon a metal tube.

FIGURE 2 shows another embodiment, in which the thread to be subjectedto the RF iield is wound on a nonmetallic cylindrical supporting member,and

FIGURE 3 is a cross-sectional View of the thread spool and the RF fieldelectrodes shown in FIG. 2.

Having now reference to FIG. 1, the numeral 1 indicates either fieldelectrode, having a cylindrical coniiguration. The thread spool 2 isplaced in the interior of said electrode. The metal tube 3, on which thethread is wound, acts as the second eld electrode. The electrodes 1 and3 are stuck into the supporting plate 4, in the interior of which theyare connected to the coaxial leads 5 and 6 through which the RF voltage,as produced by a generator, not shown in the drawing, is fed.

Having now reference to FIGS. 2 and 3, the electrodes 1 and 2 are soshaped as to exhibit annular projections 3 and 4. The thread spool 5,having a cylindrical supporting member 6 made of a non-metallicmaterial, is positioned between the electrodes 1 and 2 so that its sidesdirectly contact the annular projections 3 and 4 of the electrodes. Inaddition, to the electrodes 1 is aiiixed a cylindrical block 7, of anon-metallic material, intended to support the spool 5. The electrodes 1and 2 have, on their bottom portion, two coaxial plugs 10 and 11 whichpermit to insert into, and withdraw from the coaxial sockets 8 and 9formed in the supporting plate 12, the electrodes and spool assembly.

The coaxial plugs 8 and 9 are connected, in the interiorA of thebedplate 12 (this detail is not shown in the drawing), to the coaxialleads 13 and 14, through which the RF voltage, supplied by a generatornot shown in FIG. 2, is fed in.

This invention is not limited, however, to the embodients shown therein.Continuous treatment processes could be envisaged without departing fromthe scope of the invention. Thus, for example, the thread spools couldbe arranged in a horizontal row on a conveyor belt or another likemember which, being moved in a continuous run, causes the thread spoolssequentially to be passed between two confronting plate electrodes,energized by a RF Voltage having the specifications provided for by thepresent invention, the spatial velocity of the spool being calculated sothat each individual portion of the spool is subjected to the heattreatment for a time whose duration is roughly the one which is requiredin the treatment at standstill for producing an acceptable reduction ofthe residual shrinkage.

The invention will be now illustrated by the following practicalexamples which have no limiting value whatsoever.

EXAMPLE 1 A -filament caprolactam thread was drawn and immediately afterwound, according to the ring dressing method, on a support formed by analuminium tube having a length of 370 mms. and an outside diameter of 50mms. The denier of the drawn thread was 60 and the residual shrinkage ofthe thread was about 14%, as measured on a skin of 100 meters of threadsubjected to boiling in water for 30 minutes and then dried.

The net weight of thread wound on-the spool was about 1.6 kgs. The spoolwas wound with a gradual reduction of the stroke of the ring rail, sothat, from an initial length of stroke of 330 mms., a iinal length ofstroke of 185 mms. was obtained. The maximum diameter of the spool was115 mms. The thread spool so prepared was subjected to an RF iicld, byusing an apparatus similar to that shown in FIG. 1 hereof. The aluminiumtube forming the support of the spool was used as a eld electrode byconnecting it to a coaxial lead, whereas the second electrode was acopper cylinder having an inside diameter of 117 mms. and a length of405 mms., in the interior of which the thread spool was positioned.

The copper electrode was connected to a second coaxial lead which, alongwith the rst one, provided the energization of the electrodes. Both thealuminium tube and the copper cylinder were placed on an appropriatepedestal. The whole assembly had to be shielded in order to avoidinterferences towards the outside of the field so produced. A drivingcab, supplying a service capacity of 6 kilowatts, supplies theelectrodes with an RMS voltage of 2,000 volts at a frequency of 75megacycles. After 2 minutes the treatment was discontinued and thethread spool withdrawn.

The residual shrinkage of the thread was measured by taking samples fromthe several layers of the wound spool: the result was that the residualshrinkage varied from 2.5% to 3.5%.

EXAMPLE 2 A caprolactam thread spool having the same specifications asthat of Example 1 was subjected to a heat treatment by using the sameimplementation adopted for the previous example, the only differencebeing that a power of 4 kilowatts, an RMS voltage of 1,300 volts and afrequency of 110 megacycles were used. The duration of the treatment wasagain 2 minutes.

Thread samples, taken from several layers of the wound spool, gave aresidual shrinkage which varied within the same range as in Example l,that is, from 2.5% to 3.5%.

EXAMPLE 3 A polycaprolactam thread spool, having the samecharacteristics as in Example 1, was subjected to heat treatment byusing the same implementation as used in Example l, the only differencebeing that the driving cab had a power of 4 kilowatts and fed theelectrodes with an' RMS voltage of 2,000 volts at a frequency of 10megacycles. The implementation had not to be shielded. After 10 minutesthe treatment was discontinued and the thread spool withdrawn. Theresidual shrinkage of the thread was measured by taking samples from theseveral layers of the wound spool and the residual shrinkage was foundto vary between 3.5% and 4.5%.

EXAMPLE 4 A polyhexamethylene adipamide thread composed by 10 ilamentswas drawn and collected on a non-metallic cylindrical supporting memberslipped into a rotary drum.

The cylindrical supporting member had a diameter of mms. and a height of150 mms.

The thread, the final denier of which Was 50, Was wound on thesupporting member in the form of a spool having a width of the woundthread of 100 mms. and a thickness of mms. The net weight of the threadwas 1.8 kgs.

The thread spool so prepared was subjected to a heat treatment by meansof the implementation shown in FIG. 2.

The gap between the two eld electrodes was thus 100 mms., equal to thewidth ofthe wound spool.

The electrodes were energized by an RMS voltage of 1,500 volts at afrequency of 110 megacycles and a power of 4 kilowatts.

The treatment lasted 3 minutes.

Thread samples, taken from several layers of the wound spool, gave aresidual shrinkage which varied from 1% t-o 2%, as compared with theresidual shrinkage of 11% of the untreated thread.

EXAMPLE 5 -A lil-filament polyethylene terephthalate thread was drawnand collected on a non-metallic cylindrical support,

inserted into a rotary drum having the same size as that of Example 4.Also the size of the thread spool, the iinal denier of which was 48, wasthe same as in Example 4.

The spool was positioned in the same implementation used for treatingthe polyhexamethylene adipamide and a heat treatment was carried out byenergizing the electrodes with an RMS voltage of 1,200 volts at afrequency of 150 megacycles and a power of 6 kilowatts. The treatmentlasted 150l seconds.

Thread samples, taken from several layers of the wound spool, gave aresidual shrinkage of from 2% to 3%, as compared with an 8% shrinkage ofthe untreated thread.

While in the disclosure of this invention reference has been made tothermoplastic threads, and more particularly to those made of linearpolyamides and polyesters, the invention is not to be regarded as beinglimited to said threads only. In point of fact it Will be possible, as arule, to apply the inventive method also to other thermoplastic threadsfor which a direct heat treatment on he spool is desirable for anyreasons, such as to polyolephine, polyurethane and polyvinyl threads.

What is claimed is:

1. A method of reducing residual shrinkage of thermoplastic thread woundon a metal spool comprising the steps of positioning the wound thread asa dielectric between two electrodes, one of the electrodes being themetal spool upon which the thread is wound, and heating the thread byenergizing the electrodes with an -RF voltage.

2. The method of claim 1 including providing a metal tube as the otherelectrode.

3. The method of claim 2 including completely surrounding the woundthread with the metal tube.

4. The method of claim 1 wherein the RF voltage has and RMS value offrom 1,000 to 2,000 volts, a frequency between l0 and 200 megacycles,and a service capacity between 2 and 6 kilowatts, for a time of from 1to 10 minutes.

5. The method of claim 1 wherein the metal spool is made of aluminum.

6. The method of claim 1 characterized in that the thermoplastic threadis made of a linear polyamide.

7. The method of claim 1 characterized in that the thermoplastic threadis made of linear polyester.

References Cited UNITED STATES PATENTS 2,421,334 5/1947 Kline et al21910.81 2,433,842 1/1948 Griffin 219-10.41 2,479,351 8/ 1949 Hagopian21910t81 X 2,635,352 4/1953 Phillips 219-10.41 X 2,866,063 12/1958 Rudd219-10.81 X 2,364,294 1/1968 Garibian et al. 21910.61 X

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner U.S.Cl. X.R. 34-l; 219-10.61, 10.81

